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Full text of "The Cambridge Natural History"

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AXIS

vertebrae   are  occasionally  wholly   (Right  Whales)   or  partially
(many  Whales,  Jerboa, certain  Edentates)  welded   into a com-

FIG. 10.—Side view of axis of Dog.
x f. o, Odontoid process ; pz,
posterior zygapophysis ; s, spin-
ous process ; t, transverse pro-
cess ; vt vertebrarterial canal.
(From Flower's Osteology.)

FIG. 11.—Anterior surface of axis
of Bed Deer, x -g. ot Odon-
toid process ; pz, posterior
zygapopliysis ; snt foramen for
second spinal nerve. (From
Flower's Osteology.)

hined mass. Indications of this have even been recorded in the
human subject.
The dorsal vertebrae vary greatly in number : nine {Hyper-
oodon) seems to be the lowest number existing normally ; while
there may be as many as nineteen, as in Centetes, or twenty-two,
as in Hyrax. These vertebrae are to be defined by the fact that
they carry ribs, and the first one or two lumbars are often
" converted into" dorsals by the appearance of a small super-
numerary rib. The spinous processes of these vertebrae are
commonly long, and sometimes very long. It is only among the
Glyptodons that any of these vertebrae are fused together into a
mass.
The lumbar vertebrae, which follow the dorsal, vary greatly
in number. There are as few as two in the whale Neobalaena,
as many as seventeen in Tursiops; this group, the Cetacea,
contains the extremes. INlne lumbars are found in the Lemurs
Indris and Loris. As a rule the number of lumbars is to some
extent dependent upon that of the dorsals. It often happens that
the number of thoraco-lumbar vertebrae is constant for a given
group. Thus the Artiodactyles have nineteen of these vertebrae,
and the Perissodactyles as a rule twenty-three. A greater
number of dorsals implies a smaller number of lumbars, and of
course vice versa. The existence of a sacral region formed of a